July/August are probably the worst months to book flights from the UK due to the school summer holiday period. I always fly direct with either Thai or EVA, there's not much between them however I'd generally pick EVA over Thai
You can apply for a 60 tourist visa before travelling, they're easy to do and should not take long to be approved. It will be cheaper and easier than seeking an extension in Thailand.
You need to provide more information to get some help such as Your nationality, how long you want to stay etc.
Start by looking at your local Thai Embassy for information on visa. Most applications are an evisa now they will tell you the requirements and paper work of needed.